A porous medium domain is a portion of space that (1) is occupied by a number of phases, at least one of which is a solid phase, and (2) for which a REV can be found.

a material that has holes in it -- like swiss cheese, but these holes are microscopic in size

a material with a network of small and large holes, for example a sponge

Material that will allow water to flow through it. Sand, soil, and some types of rock are typical geologic materials that act as porous media.

A medium that has numerous interstices, whether connected or isolated.

A porous medium or a porous material is a solid (often called frame or matrix) permeated by an interconnected network of pores (voids) filled with a fluid (liquid or gas). Usually both the solid matrix and the pore network (also known as the pore space) are assumed to be continuous, so as to form two interpenetrating continua such as in a sponge. Many natural substances such as rocks, soils, biological tissues (e.g. bones), and man made materials such as cements, foams and ceramics can be considered as porous media.